P. aeruginosa is an opportunistic organism capable of colonizing skin, ear, lung and bowel. In healthy individuals, such colonization does not normally cause a problem. However, if the individual also has an underlying disorder or condition that compromises their immunity, then infection can be serious. Examples of such disorders or conditions include chemotherapy-induced immunosuppression, diabetes mellitus, cancer, AIDS and cystic fibrosis. It has been estimated that more than 70% of patients with cystic fibrosis are infected with P. aeruginosa. In these patients, P. aeruginosa infection is associated with chronic obstructive bronchitis.
Colonization of P. aeruginosa begins with attachment of the bacterium to epithelial tissues (e.g., lung epithelia). Mucoid strains of P. aeruginosa produce a mucoid exopolysaccharide (i.e., MEP or alginate) which is used by the bacterium throughout the infection. MEP is a polymer of uronic acids.
The bacterium can be relatively resistant to antibiotic therapy and innate and adaptive immune mechanisms, including antibody and complement mediated pathways. MEP is believed to be a contributing factor to immune resistance of the microbe.